1. Field of the Invention
The present invention relates to a process and an apparatus for injection-molding a foamed article, and particularly to an injection-molding process and apparatus for preparing a foamed article including a foamed core and an outer or external solid skin layer of unfoamed material.
2. Prior Art
The products of synthetic resin prepared through the known structural foam processes are generally characterized by excellent rigidity, and sound-insulating and heat-insulating capacities, together with easy moldability for the preparation of a variety of molded products having different shape and density. However, the known processes of such kind have a disadvantage that eddy or swirling patterns which would impair the visual appearance of the products are formed on the surfaces thereof due to foaming to necessitate the surface coating of the products in some cases. In order to eliminate this disadvantage, there have hitherto been developed a variety of structural foam processes and apparatuses used therefor. In one of the previously developed process, a pair of injection molding units having the same capacity is used, one of which units contains a synthetic resin for forming the outer skin layer of the product and the other of which units contains another synthetic resin for forming the core or internal foamed body of the product, and the units are operated such that the said two kinds of synthetic resins are alternately injected, for example by switching a changeover valve assembled therewith, into a mold. In this known process, a first synthetic resin injected into the cavity of the mold from the first unit through the gate of the mold contacts with the walls defining the cavity to be lowered in temperature with attendant increase in its viscosity to stagnate on the walls at the vicinity of the gate, and the following first synthetic resin mass successively fed through the gate of the cavity passes through the internal portion of the precedingly-injected first resin and then stagnates on the deeper wall portions, in other words, on the wall portions spaced at a greater distance from the gate of the cavity. During one cycle injection operation, the aforementioned successive stagnation of the continuously injected first resin material is repeatedly and continuously effected to fill the cavity. Then the change-over valve is switched to inject a second synthetic resin for forming the internal foamed body through the gate of the cavity to fill the molded body internally of the layer formed by the said first synthetic resin.
However, the viscosities of the injected first and second resins must be maintained at constant values and also the first and second resins must be injected at constant rates, in order to effect the aforementioned operations to agree with the theoretical assumption. However, in this known process, since two different resins of different state are injected through the pair of two injection units, it is extremely difficult to inject both resins while allowing them to have uniform fluidity or viscosity to effect continuous and stable injection. As a result of the switching of the valve for the alternate injections of the first and second resins, a hesitation mark, i.e. an interruption line of swirling form, is formed, or the foamed body tends to be exposed on the surface of the molded product in case where a molded product having a relatively complicated shape is molded, or a disadvantage is induced such that the foamed body does not extend to the portion remotest from the gate of the cavity.
Another known process involves an improvement in construction of the extruder for passing the molten resins onto the mold mounted on the injection molding machine, wherein the first synthetic resin for forming the outer skin layer is fed through the outer peripheral portion of the extruder and the second synthetic resin for forming the inner foamed core portion or body is fed through the inner or central portion of the extruder, whereby the first and second synthetic resins are injected into the mold simultaneously. However, the properties of the first and second synthetic resins used in this known process are different from each other, leading to a disadvantageous result that the second synthetic resin tends to be exposed on the surface of the molded product or the first and second synthetic resins are mingled or mixed at the injection step to make the process inoperative. Anyway, there has not been known or realized in the art a simple process for the production of excellent molded article composed of an outer solid or rigid skin layer surrounding an inner foamed core body.